Method and apparatus for analyzing fluids



May 14, 1963 G. J. HECHT ETAL METHOD AND APPARATUS FOR ANALYZING FLUIDs3 Sheets-Sheet 1 Original Filed July 1, 195'? INVENTORSI G. J. HECHT V.N. SMITH BY: Hvfq THEIR AGENT May 14, 1953 G. J. HEcI-IT ETAI. 3,089,382

l METHOD AND APPARATUS FOR ANALYZTNG FLUlDs original Filed July l, 1957l s sheets-sheet 2 AC= ZERO -A I.I-If2I- I FIGURE 2 FROM AMPLIFIER 43RELAY OUTPUT 45 R 44 TO AMPLIFIER 30l 32 R TO RECORDER FIGURE 3 FILTERTI Te FIXED MIRROR 73) -SAMP SIGNAL \cEI I DETECTOR DRIVEN MIRROR 76/FIGURE 5 INvENToRs:

G J. I-IEcHT v. N. SMITH THEIR AGENT May 14, 1963 G. J. HECHT ETALMETHOD AND APPARATUS FOR ANALYZING FLUIDs 3 Sheets-Sheet 3 OriginalFiled July l, 1957 mmm moDOm @A INVENTORSI @.J. HECHT v. N. SMITH BY:14.13; CM1/ THEIR AGENT 3,089,382 METHD AND APPARATUS FR ANALYZHNGFLUIDS George l. Hecht, Ei Cerrito, and Vigo N. Smith, San Leandro,Salif., assignors to Shell @il Company, New York, NX., a corporation ofDelaware Continuation of application Ser. No. 669,161, July 1, 1957.rFhis application May 26, 1959, Ser. No. 816,360 S Claims. (Ci. 88-14)This invention relates to the analysis of heteratomic fluid mixtures,and pertains more particular-ly to the method and apparatus forcontinuously effecting such analysis, automatically recording theresults thereof, and/or automatically controlling the composition, flowrate or -other parameters of said iiuid for mixtures thereof.

The system of the present invention preferably makes use of theprincipal of selective light absorption and is particularly suitable`for determining, recording and/or controlling the proportions of two-or more components in a fluid stream.

To date infrared spectroscopy has been confined mainly to the spectralregion `between 2.5 and l microns. This has been mainly due to the yfactthat an infrared analyzer capable of measuring the spectral region below2.5 microns is a very complex instrument with a complicated opticalsystem. For the purpose of describing the present invention, thespectral region which is arbitrarily talken as 'froml about 1.0 to 2.6microns will be referred to as the near infrared region.

The near infrared region has not been of great interest tospectroscopists in the past because most of the absorption 'bands inthis region are overtones of the fundamental frequencies of higherinfrared regions. There has been increasing interest in the nearinfrared as it has been found that the near infrared spectral region hasbecome attractive for use in continuous analyzing systems. This ismainly due to the `fact that the sample cells that can be used in a nearinfrared analyzer may be many times larger than those used in thepresent infrared analyzers and hence are more practical in plantinstruments. In addition, the light sour-ce used in the present nearinlfrared system is an ordinary incandescent lamp while the windowmaterials of the sample cell are either glass or fused quartz.

lt is therefore a primary object of this invention to provide a methodand apparatus for carrying out analyses of tluid bodies having two ormore components. Specifically, the present method and apparatus may beemployed in the measurement of water in various organic liquids, forexample, isopropyl alcohol; the measurement of ketones in alcohols orhydrocarbons; the measurement of alcohols in ketones or hydrocarbons;the measurement of aromatics in hydrocarbons; the measure of oletins inhydrocarbons; etc.

A further object of this invention is to provide a radiation analyzer ofhigh sensitivity, stability and discriminating power `for analyzingmulti-component fluid streams.

Another object of the present invention is to provide a simple andrugged radiation analyzer empl-oying a single optical path so as to makethe readings optically stable and thus `superior to the conventionaldouble beam analyzers wherein contaminating materials may collectunequally on the windows of the sample cells.

These and other objects -of the present invention will be understoodlfrom the following description taken with reference to the attacheddrawings, wherein:

FIGURE 1 is a diagrammatic View indicating one general arrangement ofthe parts Iforming the present system;

Patented May 14, 1963 FIGURE 2 illustrates one possible wave formproduced by the present radiation analyzer;

FIGURE 3 is a circuit diagram of the basic pulse height selector andstorage circuit used with the present analyzer;

FIGURES 4 and 5 are views of alternative arrangements of the detectionportion of the present system.

Referring to FIGURE l, reference 11 indicates a source of radiation suchas a light or lamp having a filament of suitable alloys heated to aproper temperature to emit a desired spectrum of rays when electricalenergy -is supplied to said lamp. lf the present multi-wave lengthanalyzer is to be used as a near infrared analyzer the light source maybe an automobile head-light type source. On the other hand, anultraviolet lamp may be employed if the present radiation system is tobe used as an ultraviolet analyzer. Filament voltage of the light sourcemay be regulated by a suitable transformer 12.

Arranged in front of the llamp 11 is a sample or absorption cell 13 anda radiation detector, for example a photo-conductive detector 114. Thecell is preferably made of a Isuitable metal and is provided with a pairof transparent windows 15 and 16 which are arranged in a line betweenthe light source 1l and the detector lli.

Any material transparent to radiation of the wave length being employedmay be used to make the windows 15 and 16, such as glass, quartz,lithium ll-uoride, sodium chloride, silver chloride, etc., the choice ofany particular material being governed by the region of the spectrum inwhich it is desired to operate. When operating in the near infraredspectrum from 1.0 to 2.6 microns, glass or quartz is satisfactory.Lithium fluoride windows when used in a radiation analyzer, would cutoff any radiation having a wave length of more than 6.5 microns, whi-lesilver chloride coul-d be used for wave lengths up to about 20 microns.The sample cell 13 is also provided with fluid inlet and outlet means 17and 18, respectively by which a sample of fluid to be analyzed may becirculated through the cell i3.

interposed between the light source l1 and the detector v14, on eitherside 'of the sample cell 13, is a shutter mechanism comprising a disc2t) rotatably mounted on a shaft 21 driven by an electric motor 22. Ahigh speed motor which rotates at, say, 3600 r.p.m. is preferably usedif it is desired that the radiation analyzer should possess very rapidresponse to changes in a iluid stream being analyzed. Fixedly mounted inthe rotating disc 2t) are two for more multilayer interference filters.

Multilayer interference filters normally comprise a plurality `of layersof a substance, for example, germanium, tellurium, etc. which arealternately stacked with layers of another substance, such as cryolite,on a glass plate or any other transparent dielectric plate.

The interference-type filters 2,2 and 23, known as narrow pass bandfilters, provide single wave length selection by filtering a source ofradiant energy so as to reject all radiation except that in a small passband centered about -a desired wave length. The particular tilter usedin any analyzer depends upon the -wave length that is being analyzed.The filters 22 and 23 are for different wave lengths, as will be.described hereinbelow.

The photocon'ductive detector cell 14 preferably has its photosensitiveelement made of lead suliide when the radiation analyzer of the presentinvention is being employed as a near infrared analyzer. Detector cellsof this type are sold under the trade name of Ek/tron by Eastman KodakCo. of Rochester, N Y. A 10 x 10 millimeter square lead sulfide cell ismounted in a sealed unit 14 having a quartz window 25. The cell 14 is incontact with a metal block-type heater 26 provided with a thermostat(not shown) for controlling the temperature aosaasa of the detector toil" C., preferably at a temperature higher than the ambient temperaturesat the location where the analyzer is being used. In plant locationsthis temperature may be of the order of 55 C. If it is desired to studywave lengths higher than the near infrared spectrum, detector cells madeof materials other than lead sulfide may be used, such as for example,lead telluride, lead selenide, etc. For an ultraviolet analyzer thedetector could be a photomultiplier-the high output of which wouldeliminate the need for amplifier 30. The detector 14 is electricallyconnected through an amplifier 30 to a ratio display circuit comprisinga pulse height selector 31, a switching relay 32, and thence through oneof t-wo 'capacitors 33 or 34 to a ratio recorder 35. If desired, anautomatic gain control loop may be employed between the detector 14 andthe ratio display circuit. The automatic gain control loop comprises aswitch 36, a DC. amplifier 37 and a means for providing a referencevoltage 38 such as a battery or the equivalent thereof.

Suitable means are provided for actuating the switching relay 32synchronously with the movement of the filters 22 and 23 past the lightsource `11 so that the detector 14 is connected through first onecapacitor 33 and then the other capacitor 34 to the ratio recorder 35 asfirst one `filter 23 and then the other filter 22 passes the sample cell13. One possible type of actuating means is illustrated in FIGURE 1which comprises a rotating onof chopper disc 41 which may be in the formof a semicircular opaque plate iixedly secured to the rotating shaft 21.However, better balance on the shaft is obtained when the chopper 41 isa transparent disc, preferably made of a plastic with a portion of thearea of said disc, say one-half its area, painted black.

Thus, when the chopper disc 41 is interposed between the light source 11and a second `detector 42, the black portion of the disc cuts off lightfrom the detector 42 during a portion of each rotation of disc 41 whichrotates synchronously with filter disc 20. The switching detector 42 mayibe another lead sulfide detector similar to the signal detector 14 ormay be of any type of photoelectric cell which is connected through anamplifier 43 to the switching relay 32 so as to actuate the moveablecontact of said relay 32 from one position to the other.

It is realized that any other suitable means may be used to actuate theswitching relay 32 as long as it operates synchronously with therotation of the filter disc 20. For example, in the most simple form,two pegs could be mounted on the filter disc 20 to mechanically actuatethe switching relay 32 from one position to the other on each rotationof the disc 20.

A lead sulfide photoconductive detector 14 is `employed when the presentradiation analyzer is to be used in the near infrared range since a leadsulfide detector is rugged and highly sensitive at wave lengths from thevisible to about 2.8 microns. It is necessary, however, to provide meansfor eliminating the effect of the large temperature coefficient of thelead sulfide detector, which affects the stability of the detector. Inorder to eliminate the elects of variation in detector sensitivity,self-compensation is built into the present system in the form oftwo-beam operation with a single beam detector. With this system, theratio of energies transmitted by the sample through filters of twoselected wave lengths may be detected and serves as a measure of theamount of component for which a fiuid stream is being analyzed.

The wave length (A) of one filter is selected at a point where thecomponent to be measured absorbs strongly, while the second or referencewave length (B) of the second filter is chosen where all components arerelatively weak absorbers. For example, when it is desired to analyzefor the presence of water in isopropyl alcohol, the measuring wavelength of one filter 23 is l.95 microns while the reference wave lengthof the other filter 22 is 4 1.66 microns. Alternatively, the filters 22and 23 may be chosen so that the absorption of the interferringcomponent of the liquid beam analyzed is equal at the two wave lengths.

In operation, the motor 22 turns shaft 21 causing disc 20 and filters 22and 23 to rotate at high speed between the light source of fixedintensity 11 and the detector 14. A sample of the fluid to be analyzedis circulated through, or held stationary in, sample cell 13. Light fromthe incandescent light source 11 passes as a single beam through thechopper Iwheel 20 when one of the two filters 22 or 23 is in registerbetween the source 11 and the sample cell 13. Light passing through thesample cell 13 enters window 25 and falls on the detector 14. Thedetector receives alternate pulses of light representing first thetransmission of radiated energy by the sample at one wave length (A) andthen at the other wave length (B). These pulses of transmitted energy,which represent the individual total energies of the individuallytransmitted radiations at the selected wave lengths, are converted tovoltage pulses by the detector 14 and are then amplified by amplifier 39and sorted out by the pulse selector circuit or pulse height selector31.

As the light between the incandescent source 11 and the switchingdetector 42 is cut by the ori-ofi? chopper disc 41 rotating at the samespeed as disc 20, the signal put out by said detector 42 is amplified byamplifier 43 and serves to operate switching relay 32 for switching itfrom one position to the other. The pulse height selector 31, switchingdetector 42 and switching relay 32 operate to charge the capacitors 33and 34 to voltages V1 and V2 which are representative of the two seriesof pulses transmitted by the sample at wave lengths (A) and (B),respectively, as illustrated in FIGURE 2. The ratio of the voltages V1and V2 on the storage capacitors 33 and 34 is then recorded on anyconventional potentiometer recorder. The recorded ratio of thesevoltages is a measure of the amount of component present in the fluidstream for which said fluid stream is being analyzed.

The signal wave form put out by the detector 14 has the character shownin FIGURE 2, wherein the peak pulse heights V1 and V2 are proportionalto the total intensities of the radiant energy transmitted at the signalwave length and the reference wave length, respectively. r[he basiccircuit which performs the operations of pulse selection, peakrectification and storage is shown in FIGURE 3. Relay 32 having switchcon-facts or other switching devices, such as vacuum tubes, transistors,etc., gated by the switching detector 42 in FIGURE 1 connects capacitor44 to capacitor 34 through 4diode 45 during time t1 (FIG- URE 2) andcapacitor 44 to capacitor 33 through diode 45 during time t2. The timest1 and t2 may vary by almost half the total repetition rate T as long asthe peaks of the pulses occur during connection to the appropriatecapacitor. Capacitor 44 will then charge to the average peak negativevoltage V (FIGURE 2) and 34 to V-l-V1=V1 and 33 to V-i-V2=V2. Diodes 45and 46 can be vacuum tubes or crystal junction types.

Thus, by use of the ratio display circuit the present radiation analyzeris provided with a high degree of stability despite drifting sensitivityof the detector 14 due to changes in temperature. lf desired, in orderto maintain the operation of the analyzer within a reasonable band ofamplitudes, the voltage V2 from the capacitor 34 may be compared with areference voltage Vr obtained from element 38 and the difference of thetwo voltages is amplified by amplifier 37, the signal output of which isthe bias supply for the detector 14, thus providing automatic gaincontrol to hold voltage V2 constant. The automatic gain control featurepermits operation of the analyzer which is independent of large changesin instrument parameters such as lamp brightness, detector celltemperature, window clearness, absolute tube gain, etc. The ratio of thetwo voltages, which are representative spaanse of transmissions at wavelengths A and B, depends only on, and is a true measure of, theconcentration of the desired sample component. The incorporation ofautomatic gain control to the system also increases the range of totalintensity over which the analyzer will operate. This feature alsopermits relaxation of the linearity requirements of the signal amplifier30.

Alternate arrangements of the detection portion of the present systemare shown in FIGURES 4 and 5. In FIGURE 4 a shuttle-type filter holderSti, having three filters 51, 52 and 53 mounted therein, is arranged forsliding movement in a direction normal to a line between the radiationsource 54 and the sample cell 55. Any suitable type of pneumatic,hydraulic or electrical motor 56 is operatively connected to the holder50 (as shown by broken line 57) to move the holder back and forth sothat one filter at a time is in register with said sample cell 55. Theholder 50 may be provided with means for actuating relay 53 which has anumber of contact points equal to the number of channels in the system.In FIG- URE 4 a three-channel system is illustrated which is providedwith three filters 5l, 52, 53, a three-point relay 58 and three signalstorage capacitors 6), 61 and 62, the latter beingelectrically-connected to a ratio recorder 63 adapted to compare anypair of signals and at least the signal through the reference filterwith each of the other two signals through the other filters. Relay 58is actuated by any means responsive to movement of the holder 50, as forexample, by a switch 64 which is actuated by pegs 65 on the holder 5t).The signal detector 66, amplifier 67 and pulse height selector 68 are ofthe same types described with regard to :FIGURE 1. lf desired in placeof the ratio recorder 63, the voltages on the capacitors 60, 6l, 62 maybe connected to a circuit for giving indications proportional to thetotal intensities of the rays received by the ldetector 66.

Another arrangement of filters is shown in FIGURE 5 wherein a pair offilters 70 and 7l are shown as fixedly mounted and are moved in and outof the optical path between the light source 72 and a signal detector 73by altering the optical path to pass through first one filter 70 andthen the other filter 71. The radiated energy path from the light source72 is directed to a continually moving pivotally-mounted mirror 74 whichis actuated by suitable motor means 75. in one position of the mirror,the optical path is reflected by mirror 74 through the sample cell 76,through filter 71 to be reflected off a fixed mirror 78 into the signaldetector 73. A second fixed mirror 79 is employed when the optical pathpasses through filter 70. If desired, the filters 70 and 7l and thefixed mirrors 78 and 79 may be positioned on the other side of thesample cell 76.

This application is a continuation of U.S. patent application Serial No.669,161, filed July 1, 1957, now abandoned.

We claim as our invention:

l. The method of analyzing a sample stream for the presence of acomponent therein capable of absorbing electromagnetic radiation ofselected wave lengths, said method comprising directing a single beam ofelectromagnetic Waves radiated from a radiation source of fixedintensity through said sample stream, said beam of radiation containingWave lengths within a preselected band which are absorbed by thecomponent under analysis, alternately filtering and blocking said beamto permit the passage of a narrow band of wave lengths centered on aselected reference Wave length of said radiation beam, at which wavelength there is a reduced amount of absorption, subsequently filteringsaid original beam a second time to permit passage of a second narrowband of wave lengths centered on a second selected wave length of saidradiated beam, said first selected wave length varying substantiallyfrom said second selected wave length, and measuring the ratio of theindividual total energies of the individually transmitted radiations atthe two selected Wave lengths, which ratio is indicative of thecomponent for which said sample stream is being analyzed.

2. The method of analyzing a sample stream for the presence of at leasttwo components therein capable of absorbing electromagnetic radiation ofselected wave lengths, said method comprising directing a single beam ofelectromagnetic waves radiated from a light source of fixed intensitythrough said sample stream, said beam of radiation containing wavelengths within a preselected band which are absorbed by the componentunder analysis, alternately filtering and blocking said beam a firsttime to permit the passage of a narrow band of Wave lengths centered ona selected reference Wave length of said radiation beam, subsequentlyfiltert-ing said original beam to permit passage of a number of narrowbands of wave lengths centered on an equal number of selected wavelengths of said radiated beam, said number of bands and selected Wavelengths being at least equal in number to the number of components forwhich the sample stream is being analyzed, and measuring the ratio ofthe individual total energies of the individually transmitted radiationsat each selected wave lengths to the reference wave length.

3. A radiation analyzing system comprising light source means of fixedintensity for radiating a spectrum along a single optical path, anabsorption cell positioned in said path, means I.for admitting to saidcell means a fluid mixture to be analyzed, windows in said cell alignedto permit the rays traveling along said path to traverse said cell,radiation detector means arranged in the optical path of said lightsource with said absorption cell to receive the rays traversing saidcell and put out an electrical signal proportional thereto, at least twonarrow band pass filters mounted at a point between the light source andthe detector, said filters being adapted to transmit rays of differentwave lengths, means for altering the relative position of said opticalpath and said filters whereby said rays of said optical path passthrough one of said filters at a time, light-blocking means positionedbetween said filters and mounted for movement into and out of theoptical path alternately with the filters, and total intensity measuringcircuit means electrically connected to said detector for givingindications proportional to the ratio of individual total intensities ofsaid rays received by said detector means.

4. A radiation analyzing system comprising light source means of fixedintensity for radiating a beam of electromagnetic Waves along a singleoptical path, an absorption cell positioned in said path, means foradmitting to said cell means a liuid mixture to be analyzed, windows insaid cell aligned to permit the rays traveling along said path totraverse Said cell, a radiation detector arranged in the optical path ofsaid light source with said absorption cell to receive the raystraversing said cell and put out an electric signal proportionalthereto, window means in said detector for admitting rays thereinto, atleast two filters mounted for alternate movement into and out of theoptical path at a point between the light source and the detector, eachof said filters being adapted to transmit rays or a narrow band of wavelengths centered about a preselected wave length, said preselected wavelengths of said filters varying substantially from each other,light-blocking means positioned between said filters and mounted formovement into and out of the optical path alternately with the filters,and total intensity measuring circuit means electrically connected tosaid detector for giving indications proportional to the ratio ofindividual total intensities of said rays received by said detector.

5. A radiation analyzing system comprising light source means of fixedintensity for radiating a beam of electromagnetic waves along a singleoptical path, an absorption cell positioned in said path, inlet andoutlet means for circulating through said cell means a fluid mixture tobe analyzed, windows in said cell aligned to permit the electromagneticwaves traveling along said paths to traverse said cell, aphotoconductive detector arranged in the optical path of said lightsource with said absorption cell to receive the waves traversing saidcell and put out an electric signal proportional thereto, window meansin said detector for admitting waves thereinto, at least two filtersadapted to transmit waves of a narrow band of wave lengths centeredabout a preselected wave length, said preselected wave length of onefilter varying substantially from the preselected wave length of otherfilters, means for alternately moving said filters one at a time intoand out of the optical path at a point between the light source and thedetector, light-blocking means positioned between said filters andmounted for movement into and out of the optical path alternately withthe filters, and total intensity measuring circuit means electricallyconnected to said detector for measur ing and indicating the ratio ofthe individual total energies of the transmitted waves at the wavelengths of any pair of filters.

6. A radiation analyzing system comprising light source means forradiating a beam of electromagnetic waves along a single optical path,an absorption cell positioned in said path, inlet and outlet means forcirculating through said cell means a liuid mixture to be analyzed,windows in said cell aligned to permit the electromagnet waves travelingalong said paths to traverse said cell, a photoconductive detectorarranged in the optical path of said light source with said absorptioncell to receive the waves traversing said cell and put out an electricpulse whose maximum height is proportional to the intensity of thereceived radiation, window means in said detector for admitting wavesthereinto, at least two multilayer interference filters adapted totransmit waves of a narrow band of wave lengths having a preselectedwave length in the center thereof, said preselected wave length of onefilter varying substantially from the preselected wave length of theother filter, means for alternately moving said filters one at a timeinto and out of the optical path at a point between the light source andthe detector, light-blocking means positioned between said filters andmounted for movement into and out of the optical path alternately withthe filters, and measuring circuit means electrically connected to saiddetector for measuring and indicating the ratio of pulse heights of thetransmitted waves at the wave lengths of the two filters, said circuitmeans including a pulse height selector means for separating the signalsof one pulse height from the signals of the other pulse heights, meansfor measuring the separated pulse heights, and means for comparing theseparated measured pulse heights.

7. A radiation analyzing system comprising light source means of fixedintensity for radiating a spectrum along a single optical path, anabsorption cell positioned in said path, means for admitting to saidcell means a fluid mixture to be analyzed, windows in said cell alignedto permit the rays traveling along said path to traverse said cell,radiation detector means arranged in the optical path of said lightsource with said absorption cell to receive the rays traversing saidcell and put out an electrical signal proportional thereto, at least twonarrow band pass filters mounted at a point between the light source andthe detector, said filters being adapted to transmit rays of differentwave lengths, means for altering the relative position of said opticalpath and said filters whereby said rays of said optical path passthrough one of said filters at a time, light-blocking means positionedbetween said filters and mounted for movement into and out of theoptical path alternately with the tilters, and total intensity measuringcircuit means electrically connected to said detector for givingindications proportional to the individual total intensities of saidrays received by said detector means.

8. A radiation analyzing system comprising light source means of fixedintensity for radiating a beam of electromagnetic waves along a singleoptical path, an absorption cell positioned in said path, means foradmitting to said cell means a fluid mixture to be analyzed, windows insaid cell aligned to permit the rays traveling along said path totraverse said cell, a radiation detector arranged in the optical path ofsaid light source with said absorption cell to receive the raystraversing said cell and put out an electric signal proportionalthereto, window means in said detector for admitting rays thereinto, atleast two filters mounted for alternate movement into and out of theoptical path at a point between the light source and the detector, eachof said filters being adapted to transmit rays of a narrow band of wavelengths centered about a preselected Wave length, said preselected wavelengths of said filters varying substantially from each other,light-blocking means positioned between said filters and mounted formovement into and out of the optical path alternately with the filters,and total intensity measuring circuit means electrically connected tosaid detector for giving indications proportional to the individualtotal intensities of said rays received by said detector.

References Cited in the file of this patent UNITED STATES PATENTS1,840,500 Geficken et al. Ian. 12, 193,2 2,648,253 Sweet Aug. ll, 19532,764,692 Miller Sept. 25, 1956 2,774,277 Mochler Dec. 18, 19562,775,160 Foskett et al Dec. 25, 1956 FOREIGN PATENTS 466,113 CanadaJune 27, 1950

1. THE METHOD OF ANALYZING A SAMPLE STREAM FOR THE PRESENCE OF ACOMPONENT THEREIN CAPABLE OF ABSORBING ELECTROMAGNETIC RADIATION OFSELECTED WAVE LENGTHS, SAID METHOD COMPRISING DIRECTING A SINGLE BEAM OFELECTROMAGNETIC WAVES RADIATED FROM A RADIATION SOURCE OF FIXEDINTENSITY THROUGH SAID SAMPLE STREAM, SAID BEAM OF RADIATION CONTAININGWAVE LENGTHS WITHIN A PRESELECTED BAND WHICH ARE ABSORBED BY THECOMPONENT UNDER ANALYSIS, ALTERNATELY FILTERING AND BLOCKING SAID BEAMTO PERMIT THE PASSAGE OF A NARROW BAND OF WAVE LENGTHS CENTERED ON ASELECTED REFERENCE WAVE LENGTH OF SAID RADIATION BEAM, AT WHICH WAVELENGTH THERE IS A REDUCED AMOUNT OF ABSORPTION, SUBSEQUENTLY FILTERINGSAID ORIGINAL BEAM A SECOND TIME TO PERMIT PASSAGE OF A SECOND NARROWBAND OF WAVE LENGTHS CENTERED ON A SECOND SELECTED WAVE LENGTH OF SAID